Iron blue was first discovered by Diesbach in 1704 and since that time it has been called by various names including Prussian blue, Milori blue, Chinese blue, Bronze blue, etc. These names have often indicated the shade of the iron blue ranging from a reddish to a greenish although many of these various names have lost their original meaning in recent times.
Iron blues are generally considered to have the composition Fe.sub.4 [Fe(CN).sub.6 ].sub.3 or Fe(NH.sub.4)Fe(CN).sub.6, depending on the method of manufacture. In the usual method preferred by industry in which the iron blue is not precipitated on a substrate, a solution of sodium ferrocyanide is reacted with ferrous sulfate in the presence of ammonium sulfate to form Berlin white which is then digested in sulfuric acid and oxidized with sodium bichromate or chlorate to iron blue.
For cosmetic applications, it is preferred to have the iron blue highly adherent to the surfaces of a nacreous pigment substrate in order to prevent migration of color under use conditions. For example, by combining the absorbing iron blue pigment with a yellow colored nacreous pigment, a green product can be obtained. However, merely blending the iron blue pigment with a nacreous pigment results in a product in which the iron blue is not held on the surfaces of the pigment, and migration with its resulting color changes occur.
In U.S. Pat. No. 3,951,679, it is pointed out that the conventional precipitation of absorption pigments onto nacreous pigments is unsuitable for a precipitation of iron blue because the latter forms colloidal deposits during direct precipitation, which adhere only partially or not at all on the flakes of the nacreous pigment. This leads to non-reproducible batches, to considerable difficulties during filtering, and most particularly, to a lack of nacreous effect. In order to cause iron blue to adhere to the surfaces of the nacreous pigment, the patent teaches using the device of coating an iron blue precursor on the nacreous pigment and then converting the precursor to iron blue. The precursor is a low solubility iron (II) compound since trivalent compounds are stated to lead to colloidal deposits.
The difficulties in using ferric salts were overcome in Armanini and Johnson U.S. Pat. No. 4,047,969, assigned to the assignee of the present invention. Iron blue is directly and adherently precipitated to a substrate by the addition of ferric ions to a ferrocyanide solution and adjustment of the pH.
Recently, the United States Food & Drug Administration has provided specifications concerning certain impurities in iron blue pigments. Among these specifications is a limitation that iron blue must not contain more than 10 ppm of water-soluble cyanide. Adsorption of the ferrocyanide ion can cause high cyanide levels, and the ferrocyanide ion is very easily adsorbed by iron blue because of its large negative charge. The amount adsorbed can be on the order of 0.6-0.8 mole per mole of metal ferrocyanide. This high adsorption capacity has caused great difficulty in elucidating the structure of the iron blue and defining its exact composition. In many cases, the adsorption, not only of ferrocyanide but often of other ions, is so high that the formulas which are given are only approximations of the true compositions. If the ferrocyanide ion [Fe(CN).sub.6 ].sup.-4 is desorbed, it can be oxidized to the ferrocyanide ion [Fe(CN).sub.6 ].sup.-3 which readily breaks down to form cyanide.
Manufacturers who precipitate iron blue without the presence of any substrate usually do not have difficulties in meeting the soluble cyanide specification of 10 ppm maximum. However, those manufacturers who precipitate iron blue on a substrate, especially on mica or titanium dioxide-coated mica, have had great difficulty in meeting the soluble cyanide specification for a number of reasons.
The iron blue is generally coated at a concentration of 1 to 10% on a substrate that has an extremely large surface area. The result is that the iron blue is formed in essentially a thin film and also has a large surface area. Accordingly, any reactions in which iron blue will participate can occur very rapidly with the iron blue substrate.
Iron blue is heat sensitive, and at high temperatures, it breaks down to cyanide and iron oxide. While the decomposition of iron blue is usually given as 150.degree. C., some very slight decomposition can take place at much lower temperatures. For this reason, the conventional drying of iron blue is generally carried out at about 30.degree. C. for very long periods of time, which is not practical in current pigment manufacture.
Iron blue always retains a certain amount of water, both adsorbed water and water of hydration, which it loses only at high temperatures. The presence of the small amounts of water appears to stabilize the iron blue structure. However, in the case of iron blue coatings on high surface area substrates, there are indications that the water is lost very rapidly and once lost, the breakdown to cyanide proceeds.
It will be appreciated that since the coating of iron blue on the high surface area substrates is only on the order of 1-10%, it is difficult to dry the pigment thoroughly and at the same time leave the small amounts of water necessary for stabilization. Generally, all of the water is lost because of the high surface area, and decomposition begins readily.
Additionally, iron blue is very complex in chemical structure, and some of its properties are not understood. For example, the very slight decomposition which iron blue shows under certain conditions and in some media at ambient and elevated conditions are not understood. Under some conditions, a slight amount of decomposition of iron blue leads to an increase in water-soluble cyanide.
The preparation of iron blue-coated nacreous pigments as described in U.S. Pat. Nos. 3,951,679 and 4,047,969 do not result in products meeting the 10 ppm water-soluble cyanide requirement.
It is accordingly the object of this invention to provide a new iron blue-coated nacreous pigment and a method for its production whereby the requirement of 10 parts per million or less of water soluble cyanide is achieved. This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description.